Many semiconductor devices have multiple clock signal sources available for use. If clock signals from two clock signal sources are not in phase, then switching directly from one clock source to another clock source will likely cause a short runt pulse with a clock period less than a minimum clock period. The short runt pulse could cause incorrect operation of the semiconductor device. One technique for avoiding glitches is to turn off the current clock source, then allow a couple of cycles of the clock signal from the next clock source to occur before selecting use of the next clock source.
Another technique for avoiding glitches uses a synchronizer circuit. For example, U.S. Pat. No. 6,324,652 describes use of a synchronizer circuit to avoid glitches. One issue with such synchronizer circuits is that they may require clock signals from both the current and next clock source to be running in order to switch from use of the current clock source to use of the next clock source. If the current clock signal fails prior to switching to the next clock signal, switching to the next clock signal may not take place.